1. Field of the Invention
This invention relates generally to the structure and fabrication process of MOSFET transistors. More particularly, this invention relates to a novel and improved MOSFET structure and fabrication process implemented by forming deep and narrow doping regions through doping trenches.
2. Description of the Prior Art
In a conventional vertical power MOSFET fabrication process, the requirement to form a high concentration p-region with sufficient depth below the n.sup.+ source region in order to prevent an avalanche breakdown to occur at the junction directly underneath the n.sup.+ source region generates an intrinsic limitation in shrinking the cell size. This limitation can be well appreciated by referring to a standard trenched MOSFET transistor as that shown in FIG. 1A. This trenched power MOSFET has a built-in parasitic bipolar structure with the P-body, N.sup.+ source and drain regions formed as a base, emitter and collector respectively of the parasitic bipolar. During an avalanche breakdown, due to a high current and voltage of the MOSFET, the bipolar may be incidentally turned on. As an undesirable consequence of this incidental turning on of the parasitic bipolar, the drain voltage of the MOSFET may "snap back" to or below the open-base breakdown voltage of the bipolar transistor. Due to the incidental turning on of the bipolar, as that shown in FIG. 1B, a hole-current during avalanche flows laterally underneath the n.sup.+ source region which may cause permanent damages to the transistor. Therefore, an incidental turning on of the parasitic bipolar must be suppressed and the hole-current laterally flows underneath the n.sup.+ source region must be re-directed to protect the MOSFET under high current and high voltage conditions. A method to protect a MOSFET transistor from this type of damages is to form a high concentration deep-P region, e.g., a p-short region, below the n.sup.+ source region as that shown in FIG. 1C. The deep-P region serves to reduce the current gain of the parasitic bipolar and also provides a vertical path for the hole-current to flow to the source metal contact.
A high concentration deep-P region formed with greater depth under the source region imposes several restrictions on the structure and manufacture process of a MOSFET device due to a relative large depth and lateral diffusion of the deep-P region. First, an alignment step and long diffusion thermal cycle are required to form the deep-P region at the center of the cell as that shown in FIG. 1C. The requirements of alignment and the longer thermal cycles in forming the deep-P region increase the complexity and costs of the MOSFET manufacture. Secondly, the cell size cannot be easily reduced due to the lateral diffusion of the deep-P region when this region is formed with greater depth.
Therefore, there is still a need in the art of transistor fabrication, particularly in MOSFET design and fabrication, to provide a structure and fabrication process that would remove these limitations with improved device structure and fabrication processes.